1. Field of the Invention
The present invention relates to a method for controlling a dynamo-electric machine, more particularly an electric motor, coupled to a reciprocating mechanical system having a large inertia so as to reduce stress in the system and minimize electric power consumption. More particularly, the method of the present invention provides a control system which cycles motor current from a dwell status to a operating torque and current status.
2. Description of the Prior Art
While the present invention is not so limited, it is particularly useful in the control of an electric motor for operating a pump of the type commonly referred to in the art as a walking-beam oil well pump. In the operation of such a pump, a beam rocks back-and-forth about a pivot which is generally but not always located at the center of the beam. One end of the beam is connected to the crank arm of a drive which is, in turn, driven by a motor. The other end of the beam is connected by a pitman tail through a cable to a sucker or polished rod assembly extending to a pump in the bottom portion of an oil well. The instantaneous crank pin and tail bearing locations determine the angle of the pitman and, therefore, the angle along which the force must be applied. The pitman angularity also causes an increase in the force required to balance the load. With the beam horizontal, equal vertical forces at opposite ends of the beam will balance the load on the beam, assuming a centrally located pivot. However, the force applied to the beam by the pump in normal operation changes in a non-linear manner with the position of the beam. Generally, however, as the pitman moves downward, a smaller force is imposed on the beam; as when the pitman moves upward, a larger force is imposed the beam. The mechanical drive system for the beam similarly undergoes cyclically changing non-linear loading which vary with the position of the walking beam. During the up stroke, a plot of load versus displacement shows a rapid increase (i.e., at a relatively steep slope) while the polished rod and column of oil are accelerated upwardly. During the down stroke, the plot of load decreases at a relatively less steep slope as the pump piston moves downwardly under the weight of the rod and oil column. The polished rod, which is also a part of the mechanical system, is typically a series of successively decreasing diameter rods and may undergo an effective length change (strain) due to both static and dynamic loading (stress) on the rod including, for example, a change in direction of the rod s movement in its reciprocating motion as well as the load imposed on the rod by the pumping action. It has been standard practice to observe certain aspects of pumping operation by an analysis using a dynamometer card which shows load versus displacement at the rod. Rod loads may intentionally or unintentionally be dramatically increased by operation at or near resonance of the rod string. In certain applications, such resonance may impose unacceptably high loads on the rod, while in other circumstances, e.g. where static rod loads are greatly below rated loading, such resonant operation can desirably utilized, for example, to reduce electrical power consumption. Since the rod string can be as much as two miles long, and oil viscosity can vary, a great variation may be observed in both the resonant frequency and Q or sensitivity to resonance in pumping systems. If the rod string is long, care must also be taken to avoid driving the pump above the frequency range at which it can respond, since doing so will reduce the stroke volume of the pump which, in turn, will reduce the well output.
It is conventional to use a NEMA Class D motors to drive beam pumps. Since such motors have high slip at rated load, motor speed will drop off appreciably with load increases permitting utilization of stored energy provided by a flywheel which is incorporated into the mechanical system of the beam pump drive. Since the motor provides a large starting torque (up to 275% of full load torque) while requiring a relatively low starting current, Class D motors are well suited for heavy duty applications; however, power consumption by such motors is necessarily larger than NEMA Class B or Class C motors.